This invention relates to liquid, highly alkaline cleaners having a high solids content that are useful in such applications as in the production of finished steel.
For years, many industries have used liquid alkaline cleaners, normally based on sodium or potassium hydroxide, for various applications such as laundry, textile, maintenance and metalworking processes. A specific example of such an application is the alkaline cleaning of steel strip in the steel industry. Operations such as continuous and batch annealing, galvanizing and electrolytic plating require the complete removal of cold rolling lubricant residuals as the first step in their processes. In general, liquid cleaners have been preferred to powdered formulations for their ease of handling and inherent safety. Previous to this invention, the active content of these cleaners has been limited to approximately 50% by weight, with the balance composed of water. More highly concentrated versions of this class of cleaners are desirable as it reduces the volume of concentrate required to obtain a given cleaner bath concentration. This in turn reduces the end cost of the cleaner by minimizing the manufacturing, shipping and handling cost components. This invention permits the manufacture of liquid alkaline cleaners that contain as high as 80% active ingredients; furthermore, due to the basic chemical and physical properties upon which they are based, virtually unlimited combinations of normally employed alkaline cleaner additives can be used depending upon the intended application and performance specifications.
In addition to percent solids limitations, one of the major problems previous to this invention was the relatively low concentrations of inorganic alkaline silicates that could be incorporated into a high alkalinity liquid formula without destabilizing or gelling the cleaner. For example, using the conventional preparation procedure for a concentrated alkaline liquid which involves dissolving the caustic insoluble ingredients into a water based premix, adding the premix to the caustic base of the cleaner (normally 50% NaOH), mixing the combination for a specified time and set of conditions, and then sometimes homogenizing the final product by various mechanical means, the maximum amount of silicate that may be included has been approximately 5% on a SiO.sub.2 basis.
Inorganic alkaline silicates as a general class are important ingredients for alkaline cleaning for at least two major reasons. First, in all alkaline cleaning applications, silicates are relatively inexpensive additives, yet exhibit good detergency, soil-suspension and surface-active properties. Where permitted, the inclusion of silicates in a formulation can significantly improve the cost-performance of an alkaline cleaner. Secondly, in metal cleaning, particularly for electrolytic cleaning, highly silicated alkaline cleaners deposit a thin layer of silicate onto the metal surface. This film is important in the cleaning of strip steel prior to batch annealing, where the silicate layer helps prevent the welding of adjacent coil laps during batch anneal -- a defect appropriately called "Stickers" in the industry vernacular.
Prior to this invention any cleaning operation requiring or desiring the use of a high silicate content, highly alkaline cleaner was forced to use a powdered form of cleaner. This invention provides a stable, highly silicated, high solids liquid alkaline cleaner composition and a method for manufacturing the composition. Using the method of this invention, a liquid alkaline cleaner can be produced with a SiO.sub.2 content of 0-30%. By substituting phosphates, borates, carbonates or other cleaning additives that are insoluble and inert in concentrated liquid caustic solutions, almost any desired alkaline cleaning formulation can be produced in a highly concentrated liquid form.
The conventional manufacturing methods for the production of concentrated liquid alkaline cleaners involve the dissolution of the caustic insoluble ingredients, such as the organic additives (surfactants, chelating agents, foam controls, etc.) and the inorganic additives (silicates, phosphates, borates, etc.), into water as a premix. This conventional method cannot be used with high levels of the inorganic additives as these additives precipitate and the entire product then thickens beyond an acceptable point, sometimes resulting in complete solidification, when the premix is added to the caustic base and blended. Furthermore, using the quantity of water required to completely dissolve the additives in the premix severely limits the maximum attrainable solids content of the formulation. As an alternative conventional method of preparation, the addition of commercially available additives to the caustic base prior to premix addition or to a final blended product has also been largely unsuccessful, with the resulting blends having been found to be too unstable or thick to be practical.